6.004 http://6004.csail.mit.edu/currentsemester/[12/1/2009 9:20:52 PM] Fall 2009 Announcements Handouts Tutorial problems Past Quizzes On-line assignments Course info Course description Course objectives Course calendar Lab hours Courseware Suggestions Suggestion Box Previous terms Lab Queue WIKI (NEW! Lab hin Virtual memory indicates problems that have been selected for discussion in section, time permitting. Problem 1. Consider a virtual memory system that uses a single-level page map to translate virtual addresses into physical addresses. Each of the questions below asks you to consider what happens when one of the design parameters of the original system is changed. A. If the physical memory size (in bytes) is doubled, how does the number of bits in each entry of the page table change? increases by 1 bit. Assuming the page size remains the same, there are now twice as many physical pages, so the physical page number needs to expand by 1 bit. B. If the physical memory size (in bytes) is doubled, how does the number of entries in the page map change? no change. The number of entries in the page table is determined by the size of the virtual address and the size of a page -- it's not affected by the size of physical memory. C. If the virtual memory size (in bytes) is doubled, how does the number of bits in each entry of the page table change? no change. The number of bits in a page table entry is determined by the number of control bits (usually 2: dirty and resident) and the number of physical pages -- the size of each entry is not affected by the size of virtual memory. D. If the virtual memory size (in bytes) is doubled, how does the number of entries in the page map change? the number of entries doubles. Assuming the page size remains the same, there are now twice as many virtual pages and so there needs to be twice as many entries in the page map. E. If the page size (in bytes) is doubled, how does the number of bits in each entry of the page table change? each entry is one bit smaller. Doubling the page size while maintaining the size of physical memory means there are half as many physical pages as before. So the size of the physical page number field decreases by one bit. F. If the page size (in bytes) is doubled, how does the number of entries in the page map change?

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6.004 http://6004.csail.mit.edu/currentsemester/[12/1/2009 9:20:52 PM] there are half as many entries. Doubling the page size while maintaining the size of virtual memory means there are half as many virtual pages as before. So the number of page table entries is also cut in half. G. The following table shows the first 8 entries in the page map. Recall that the valid bit is 1 if the page is resident in physical memory and 0 if the page is on disk or hasn't been allocated. Virtual page Valid bit Physical page

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